1. Field of the Invention
The present invention relates to an electrode structure formed on a p-type III-V compound semiconductor and a method of manufacturing the same.
2. Related Background Art
A nontoxic AuZn-based metal having a low contact resistance has been conventionally used as an electrode material to form an electrode structure formed on a p-type III-V compound semiconductor. In this case, (1) an AuZn alloy is used as a deposition material; and (2) Au and Zn are deposited independently.
When an AuZn alloy is used, their vapor pressures are different from each other, and it is difficult to control a composition of a metal film deposited on the semiconductor. In this case, adhesion strength between the AuZn alloy and the semiconductor becomes insufficient. In order to solve this problem, a sandwiched structure of Au/Zn/Au (FIG. 1A) is formed to increase the adhesion strength between the alloy and the semiconductor.
Wiring bonding pads are required to connect semiconductor elements and an external circuit by wire bonding. In this case, it is preferable to integrally form the wire bonding pads on an electrode structure so as to reduce stray capacitances of the elements. An electrode structure (FIG. 1B) having a thick AuZn alloy layer having a thickness of 0.5 .mu.m to 2 .mu.m is available as a wire bonding pad (Japanese Patent Laid-Open No. 54-69979).
In order to suppress degradation caused by an irregular alloy reaction occurring during a high-temperature operation of a semiconductor element using Au as an electrode material, Ti and Pt layers are alternately stacked on AuZn, and Au is then formed on the resultant structure, thereby obtaining an electrode structure (FIG. 1C) (Japanese Patent Laid-Open No. 62-155562).
According to a conventional technique shown in FIGS. 1A and 1B, although the stray capacitances of the elements can be reduced, formation of a thick Au wire bonding film on the Au/Zn/Au layer or the AuZn alloy layer causes an alloying reaction between the semiconductor and the thick Au film. As result, element characteristics and wire bonding strength are degraded.
The thickness of the Au layer formed on a semiconductor influences a contact resistance. Taking this influence into consideration, the Au layer is formed to have a minimum thickness (e.g., 10 to 60 nm).
It is conventionally deemed that the thickness of the Au layer formed on the Zn layer or the thickness of the AuZn alloy layer does not influence the contact resistance.
According to the experimental result, however, it was found that the thickness of the Au layer formed on the Zn layer and the thickness of the AuZn alloy layer caused changes in resistance of the electrode layer although this was not taken into consideration in the conventional technique.
FIG. 2 shows an experimental result representing a relationship between the specific contact resistance and the thickness of the Au layer formed on the Zn layer in an electrode having a three-layered (i.e., Au/Zn/Au) structure. According to this experiment, when the thickness of the Au layer is decreased, the resistance is increased.
According to the prior art technique shown in FIG. 1C, degradation of element characteristics caused by an alloying reaction between the semiconductor and the thick Au bonding pad layer can be prevented while a low ohmic contact resistance is assured by AuZn. However, Pt can hardly be selectively etched, and its formation method is limited to a specific one. Since Pt itself is expensive, the manufacturing cost is undesirably high.